Technical Field
Embodiments of the invention relate generally to medical diagnostic imaging. Particular embodiments relate to attenuation correction of positron emission tomography (“PET”) images, using data concurrently obtained from magnetic resonance imaging (“MRI”).
Discussion of Art
PET scanners use one or more rings of scintillators or other detectors to generate electrical signals from gamma rays (photon pairs) that are produced from the recombination of electrons, within a target material, e.g., human body tissue, and positrons, emitted from decay of a radionuclide packaged in a tracer compound. Typically, recombination events occur within about 1 mm from the radionuclide decay event, and the recombination photons are emitted in generally opposite directions to arrive at different detectors. Paired photon arrivals that occur within a detection window, usually less than a few nanoseconds apart, are counted as indicating a recombination event. On this basis, computed tomography algorithms are applied to the scintillator position and detection data in order to locate the various recombination events, thereby producing three-dimensional images of the tracer disposition within the target material.
Typically, the tracer compound is a liquid analogue to a biologic fluid and the radionuclide is disposed primarily in body tissues that make use of the biologic fluid. For example, a common form of PET makes use of fludeoxyglucose (18F), which is analogous to glucose with the 18F radionuclide substituted for one of the glucose hydroxyl groups. Importantly, fludeoxyglucose is preferentially absorbed by brain matter, by the kidneys, and by growing cells, e.g., metastasizing cancer cells. As a result, PET is frequently used for oncologic studies, for localizing particular organs, and for studying metabolic processes.
One potential challenge in obtaining optimal PET image quality is that gamma rays, in the energy spectrum produced by positron-electron interactions, are easily attenuated by typical body tissues and are differently attenuated by different types of body tissue. As will be appreciated, varying attenuation can diminish statistical confidence in the locations of recombination events, thereby making the computed image “fuzzier” than is desirable. Accordingly, it is desirable to provide means for attenuation correction (“AC”). This is particularly desirable and challenging for patients who have highly radiopaque inclusions, such as cobalt-chromium joint replacements or nitinol stents.
Currently, computed tomography (CT) scanning is used concurrently with PET in order to obtain a model of tissue density that is suitably accurate for AC of the PET image. CT scanning utilizes x-rays to generate images. It is desirable, however, to provide a mode of PET attenuation correction that is not reliant on x-ray exposure.